grumble skuzz/propolis 2N2222A question

[drgonzo1969]

is there an ideal hfe for the 2N2222's in this circuit? i didnt see any notes in the build docs. I have a bag of bc108 ranging from 114 hfe to 277 hfe

http://effectslayouts.com/wp-content/uploads/2019/08/Grumble-Skuzz-Build-Doc.pdf

https://docs.pedalpcb.com/project/Propolis.pdf

 

[Bricksnbeatles]

This is a circuit where it definitely makes a difference, but it’s hard to say what’s “ideal”

My recommendation is to socket them and test out some different combinations

 

[neiltheseal]

I just matched the octave transistors as close on hfe as possible.

I’d recommend sockets. I wish I did. Would be good to have more gain than I have

 

[Kroars]

is there an ideal hfe for the 2N2222's in this circuit? i didnt see any notes in the build docs. I have a bag of bc108 ranging from 114 hfe to 277 hfe

http://effectslayouts.com/wp-content/uploads/2019/08/Grumble-Skuzz-Build-Doc.pdf

https://docs.pedalpcb.com/project/Propolis.pdf

The magnificent Chuck made mention of transistor needs in the Propolis, in an unrelated post. Helped me tremendously with that circuit. I’d go a bit further and read the thread as well. Also, if you haven’t yet, tons of great info in the Boneyard.

2. Differential-transistor octave circuits
There are five types of octave-up circuits on PedalPCB. One of those circuits uses a pair of transistors to perform frequency doubling. An example is the Propolis.

Q3 and Q4 act as a full-wave rectifier, switching between two copies of the signal that are 180° out of phase. Without getting into the finer details of how or why this works, suffice it to say the two copies need to be identical for the strongest octave-up content. for that to happen, Q3 & Q4 need to be matched for Vbe so that they turn on and off symmetrically. In addition, some other parts need to be matched, specifically R7 & R8, R9 & R14, R10 & R13. If you use 1% resistors, you don't need to match them by hand. If you use 5% or 10% carbon comp resistors, get out your DMM. Also, Q2 needs to have hFE >100 so that the signals at the emitter and collector are the same amplitude.

From:

What's All This Matching Stuff About? - part 1

People talk about matching transistors, but what do they mean? There are a few instances in pedal circuits where matching transistor parameters to each other or to circuit requirements is essential for proper performance. 1. FET-based Phase-Shifters Many Phase Shifters used JFETs as...

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